The first implication of the attack is on Nairobi's urban development. Will the city choose to become more open or more closed? With its population set to double to eight million in the next decade or so, design choices are powerfully relevant. The poor are already excluded from prosperous life by security arrangements. The temptation will be for more reinforcement. But if walls get higher, and those who drive cars between fortified points are further separated from those who go on foot and live in crowded places, the result will be a metropolis that is uncompetitive and unsafe.